The present invention relates to a power transmission device having an electromagnetic clutch comprising a solenoid coil, a coil housing disposed surrounding the solenoid coil, an armature plate disposed facing the side of the coil housing, and a clutch mechanism, wherein the current flowing to the solenoid coil is controlled so as to control the clamping of the armature plate to the coil housing, and the clamping force acting on the armature plate is made to act on the clutch mechanism via a cam mechanism so as to control the engagement of the clutch mechanism.
Various electromagnetic clutches of this type were known in the past, and have been disclosed in Japanese Laid-Open Patent Applications H10-194004 and 2000-240685 and elsewhere. The electromagnetic clutches disclosed in these publications are used as differential mechanisms in automobile axles. For instance, in Japanese Laid-Open Patent Application H10-194004, differential clutch mechanisms each composed of an electromagnetic clutch are installed on the left and right of a final reduction mechanism consisting of hypoid gears in a rear axle, and a differential action is achieved and drive is switched between two- and four-wheel-drive modes by engaging and disengaging these left and right differential clutch mechanisms.
A differential clutch mechanism consisting of an electromagnetic clutch with a structure such as this comprises a wet-type multi-plate clutch mechanism, a solenoid mechanism, and a ball cam mechanism. The solenoid mechanism comprises an armature plate facing a coil housing provided around a solenoid coil, the armature plate is linked to the input side of the multi-plate clutch mechanism, and the coil housing is linked to one end of the ball cam mechanism. The other end of the ball cam mechanism is linked to the output side of the wet-type multi-plate clutch mechanism and provides a thrust force for engaging the clutch mechanism.
With this differential clutch mechanism, current is sent to the solenoid coil to generate a magnetic force which clamps the armature plate to the coil housing, and this causes the coil housing to rotate along with the armature plate, so that one end of the ball cam mechanism rotates along with the input side of the clutch mechanism. Because the other (second) end of the ball cam mechanism here is linked to the output side of the clutch mechanism, if there is a rotational difference between the input and output of the clutch mechanism (such as when the rear wheel rotation is different with respect to the rotation on the axle drive side), the second end will be rotationally driven with respect to the first end of the ball cam mechanism, a thrust force in the engagement direction will be imparted from the second end to the wet-type multi-plate clutch mechanism, and the differential clutch mechanism will be engaged.
With this differential clutch mechanism, however, even in a state in which current is sent to the solenoid coil and the armature plate is clamped to the coil housing, the ball cam mechanism will be actuated and the wet-type multi-plate clutch mechanism will be engaged only when there is a difference in the input and output rotation as discussed above. If there is no rotational difference, there will be no thrust force from the ball cam mechanism to engage the wet-type multi-plate clutch mechanism. Consequently, if there is drive from the input side of the differential clutch mechanism and the input rotation speed Nin is greater than the output rotation speed Nout, such as during acceleration, a thrust force from the ball cam mechanism will act in the engagement direction on the wet-type multi-plate clutch mechanism, so that the latter is engaged. Conversely, even if the accelerator pedal is released during driving, so that the input rotation speed Nin drops below the output rotation speed Nout, a thrust force from the ball cam mechanism will act in the engagement direction on the wet-type multi-plate clutch mechanism, so that the latter is engaged.
The direction in which a rotational difference occurs between the input and output members when Nin greater than Nout is opposite from that when Nin less than Nout, and the direction in which one end of the ball cam mechanism rotates with respect to the other end is also opposite in these two cases. Accordingly, when the accelerator pedal is released to change from a state of acceleration to one of deceleration, for instance, the rotational drive force that was acting on the other end in the ball cam mechanism in the acceleration state is temporarily released and the wet-type multi-plate clutch mechanism is disengaged, and when there is a transition to a deceleration state, a rotational drive force in the opposite direction acts on the other end, a thrust force in the engagement direction is exerted on the wet-type multiplate clutch mechanism from the ball cam mechanism, and this clutch mechanism is again engaged. Therefore, it is preferable if the direction of the rotational drive acting on the ball cam mechanism is reversed simultaneously with a transition from an acceleration state to a deceleration state, affording a smooth transition to a deceleration state.
However, in a state in which the wet-type multi-plate clutch mechanism is engaged upon receiving a thrust engagement force from the ball cam mechanism in an acceleration state, the armature plate is clamped to the coil housing by the magnetic force generated when current is sent to the solenoid coil, and torque is transmitted between the input and output members through the power transmission paths on either side of the ball cam mechanism. Specifically, the power transmission path going from the armature plate and the coil housing to which it is clamped through the ball cam mechanism is in parallel with the power transmission path through the wet-type multi-plate clutch mechanism. Accordingly, when there is a transition from this state to a deceleration state, the ball cam mechanism is temporarily locked, and this torque-locked state of the ball cam mechanism is suddenly released only when the transition to a deceleration state proceeds further and the armature plate slides with respect to the coil housing. This results in a reversal in the direction of the rotational drive force acting on the ball cam mechanism, and in the re-engagement of the wet-type multi-plate clutch mechanism, but a problem here is that the direction of the rotational drive force acting on the ball cam mechanism may change sharply all at once, resulting in lurching of the vehicle.
It is an object of the present invention to provide a power transmission device with which a smooth transition can be made between a state in which the electromagnetic clutch is engaged under conditions of Nin greater than Nout, such as in the transition from an acceleration state to a differential clutch mechanism, or vice versa, and a state in which the electromagnetic clutch is engaged under conditions of Nin less than Nout.
It is a further object of the present invention to provide a power transmission device with which a smoother ride can be achieved without any delay in the switching of the direction of drive [force] acting on the cam mechanism in the above transition.
In the present invention, an electromagnetic clutch has a solenoid coil, a coil housing disposed surrounding the solenoid coil, an armature plate disposed facing the side of the coil housing, and a clutch mechanism (in an embodiment, for example, a mechanism comprising a clutch housing 52, separator plates 53, clutch plates 54, a pressure plate 55, and so forth), the armature plate is linked to a rotation input member (such as a clutch housing 52 in the examples), the rotation input member and a rotation output member (such as a left or right left side shaft 60 in the examples) are engaged and disengaged by the clutch mechanism, the current flowing to the solenoid coil is controlled so as to control the clamping of the armature plate to the coil housing, and the clamping force acting on the armature plate is made to act on the clutch mechanism via a cam mechanism (such as a ball cam mechanism 65 in the examples) so as to control the engagement of the clutch mechanism. The above-mentioned cam mechanism comprises a first cam plate linked to the coil housing, a second cam plate that is linked to the rotation output member and imparts an engagement thrust force to the clutch, and cam balls that are disposed in cam grooves formed in the first and second cam plates and are sandwiched between the first and second cam plates. A first rotational backlash amount BLm occurring between the rotation input member and the rotation output member in a rotation transmission system in which the clutch mechanism is interposed with the clutch mechanism in an engaged state, and a second rotational backlash amount BLp occurring between the rotation input member and the rotation output member in a rotation transmission system in which the cam mechanism is interposed in a state in which the cam mechanism is fixed and the armature plate is clamped to the coil housing, are set such that BLm greater than BLp.
If a power transmission device having an electromagnetic clutch is structured as above, then the difference in the above-mentioned backlash amounts can be utilized in a transition from an acceleration state to a deceleration state so that the cam mechanism is released quickly, without any delay, and torque lock can be prevented, and a smooth transition can be made to a deceleration state with no lurching.
In another possible structure, the clutch mechanism comprises a clutch housing that constitutes the rotation input member, a plurality of separator plates and clutch plates disposed inside the clutch housing and arranged alternating in the axial direction, a pressure plate provided so as to cover the plurality of separator plates and clutch plates, and a clutch hub disposed on the inner peripheral side of the plurality of separator plates and clutch plates, with either the separator plates or the clutch plates being linked to the clutch housing and the other being linked to the clutch hub, and the clutch hub being linked to the rotation output member, and the first rotational backlash amount BLm is determined by the sum of the rotational backlash amount BLm(1) of the linked portion of the clutch housing and either the separator plates or the clutch plates, the rotational backlash amount BLm(2) of the linked portion of the clutch hub and the other of the separator plates or clutch plates, and the rotational backlash amount BLm(3) of the linked portion of the rotation output member and the clutch hub.
Meanwhile, the second rotational backlash amount BLp is determined by the sum of the rotational backlash amount BLp(1) of the linked portion of the rotation input member and the armature plate, the rotational backlash amount BLp(2) of the linked portion of the coil housing and the first cam plate, and the rotational backlash amount BLp(3) of the linked portion of the second cam plate and the rotation output member. In this case, the coil housing and the first cam plate may be press-fitted and spline engaged, so that the rotational backlash amount BLp(2)=0.
It is preferable if, when the clutch mechanism is engaged by the cam mechanism, the amount of rotation BLc of the cam mechanism produced by deformation of the clutch mechanism is taken into account so that BLm greater than (BLp+BLc). If so, then even when deformation occurs in the clutch mechanism, the above-mentioned difference in the backlash amounts can be utilized in a transition from an acceleration state to a deceleration state so that the cam mechanism is released quickly, without any delay, and torque lock can be prevented.
The power transmission device pertaining to the present invention can be disposed inside an axle that transmits drive force to left and right wheels.
In this case, the power transmission devices pertaining to the present invention can be disposed symmetrically on the left and right of a final reduction driven gear that constitutes the axle, providing a differential action and switching between two- and four-wheel-drive modes.
Alternatively, the power transmission devices pertaining to the present invention may be disposed ahead of a final reduction driven gear that constitutes the axle, and perform switching between two- and four-wheel-drive modes.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.